Backpressure control is critical to maintaining process conditions. However, with solid-liquid slurries, clogging of valves and orifices is a challenge. In addition, back pressure control valves cannot respond quickly enough and completely reseal to avoid bleed-through. Process pressure variations must be minimized to maintain process control. Thus, it would be beneficial to develop an efficient and reliable means for handling fouling fluids, such as thick solid-liquid slurries of lignocellulosic biomass and its components, under high pressure that minimize clogging, including, but not limited to those processed with compressible supercritical or near-critical fluids. The apparatus of methods of the present invention are directed toward these, as well as other, important ends.